Summary Chemotherapy-induced peripheral neuropathy (CIPN) occurs in up to 75% of patients who receive cytotoxic agents such as paclitaxel (PTX), and is a major reason to discontinue chemotherapy. These patients experience pain, sensitivity to cold, and imbalance. PTX induces CIPN-related pain by activating Toll-like receptor 4 (TLR4) on monocytes, which induces expression of pro-inflammatory cytokines. Targeting TLR4 may be risky due to the role of TLR4 in immune defense against cancer. Interestingly, TLR4 activation triggers inositol-requiring enzyme 1 alpha-X-Box Binding Protein 1 (IRE1?-XBP1) signaling in immune cells. The inhibition of IRE1?-XBP1 enhances PTX antineoplastic effects by reducing pro-inflammatory factors, suggesting that it could be an attractive target to prevent CIPN and improve the efficacy of PTX for cancer. We have confirmed that PTX causes IRE1?-XBP1 activation and induces a pro-inflammatory phenotype in primary human leukocytes. Notably, leukocytes from mice lacking IRE1?-XBP1 specifically in immune cells (Ern1/Xbp1f/f-Vav1cre) do not display this pro-inflammatory phenotype upon PTX exposure. These conditional knockout (cKO) mice exhibit reduced cold allodynia and hind paw unbalance in a model of neuropathic pain (partial sciatic nerve ligation - PSNL). Additionally, through transcriptomic analyses we found that IRE1?-XBP1 signaling in leukocytes is critically required for the induction of prostanoids and cytokines that have been associated with CIPN. Therefore, we hypothesize that PTX promotes CIPN by activating IRE1?-XBP1 signaling in leukocytes, and that targeting this pathway could be used to prevent CIPN. We will accomplish the following specific aims: 1) Define the role of immune-intrinsic IRE1?-XBP1 in PTX-induced CIPN. We will administer PTX to mice with leukocyte-specific deletion of IRE1?-XBP1 (cKO) as well as their wild type (WT) counterparts. We anticipate that cKO mice will be protected from PTX-induced CIPN related behaviors. Then, we will treat WT mice with selective IRE1? inhibitors (MKC8866 or KIRA8) in order to pharmacologically prevent PTX-induced CIPN behaviors. These studies will define the feasibility and therapeutic potential of targeting IRE1? for CIPN. 2) Establish how PTX influences IRE1?-XBP1 signaling in immune cells to drive CIPN. We will exploit the ER stress-activated indicator (ERAI) transgenic mouse, whose cells express a yellow fluorescent protein variant (Venus) when IRE1? is activated. Immunofluorescence will be used to identify ER-stressed leukocytes in the blood, spleen, sciatic nerves, and dorsal root ganglia during CIPN. Leukocytes will be sorted according to reporter positivity and analyzed via RNA-Seq, immunophenotyping, and functional assays. These experiments will unearth how PTX- induced IRE1?-XBP1 activation influences leukocyte function throughout CIPN progression. Our team of neuro- oncologists, internist clinicians, and basic scientists with expertise in pain biology, neuroimmunology, immunology, cancer biology, IRE1?-XBP1, genomics and bioinformatics is uniquely positioned to test this innovative hypothesis and contribute to the development of novel non-narcotic treatments for chronic pain.